In the “long term evolution” (LTE) standard, reference signals (also known as pilot signals) are constantly broadcast by the base stations. The user terminals receiving them use them to assess the properties of the downlink channel. The LTE system uses different kinds of reference signals, sent from different antenna ports and performing different tasks. These include cell-specific reference signals (or common reference signals), user terminal-specific reference signals (or demodulation reference signals), positioning reference symbols and channel state reference signals.
User terminals estimate the downlink channel characteristics in order to perform equalization before demodulation of the data symbols. If the UE is moving, there will be time and frequency domain effects as follows:                The time domain effect is caused by multipath reflections, whereby the reflections cause disturbances in the amplitude and phase of the signals. Such reflections come from surfaces near to the UE and more distant from the UE, and the difference in time of arrival of the reflected signals generally increases with high eNodeB transmit powers and longer distances between the eNodeB and UE.        The frequency domain effect is caused by Doppler shift, whereby the frequency of the received signal at the eNodeB will be shifted down if the UE is moving away from it and shifted up if the UE is moving towards it.        
This combination of multipath reflections and the Doppler effect produce an effect known as Doppler spreading. So that the UE can correct for these impairments, the base station repeats the reference signals at regular intervals of time (typically 0.5 ms) and of frequency (typically 100 kHz), spreading across the transmitted bandwidth, to allow a UE to detect sufficient reference signals to reconstruct the channel. The reference signals can be thought of as sampling the channel in both the time and frequency domain. The user terminal uses the standard properties of the reference signals to calculate the amplitude and phase correction that should be applied to every non-reference symbol, and it interpolates between the reference signals as part of the calculation.
The standard LTE reference signal pattern is suited to macro-cellular systems where the UE can receive reflections from many kilometers away (a 3 km round trip is a delay of approximately 1 millisecond), and still rebuild the time domain aspects of the channel, and where the UE can be travelling at 300 km/hr and still rebuild the frequency domain aspects of the channel. The reference signals are transmitted all of the time and comprise about one seventh of the maximum output power of the base station.
For small cells, such as indoor femtocells, both the range and speed of user terminals are much smaller. It is therefore possible to reconstruct the channel in both the time and frequency domain with a very much smaller number of samples or reference signals, which reduce the power requirements of the base station, and increases scope to mitigate reference signal pollution, because each base station is using less spectrum for its reference signals.
Significant power savings and efficiency improvements can be realized by adapting the reference symbol density dynamically in this way. Examples are described in, for example, U.S. Patent Application Publication No. 2016/0020877. However, for this to be done efficiently, the base station needs to be able to determine what reference signal density is appropriate. One factor in determining a suitable reference signal density may be the degree of difficulty expected to be experienced by mobile terminals in initially acquiring the channel transmitted by the base station.